1. Field of the Invention
The present invention relates to a liquid-crystal display (hereinafter, LCD) having the capability of a tablet in which a position of a stylus in contact with a display screen is detected using an electric signal developed at the stylus through electrostatic coupling between the stylus and electrodes in the LCD.
2. Description of the Related Art
In recent years, personal computers and word processors having the capability of a stylus input (tablet) unit have begun to prevail. This has facilitated the tendency toward more compact and lightweight information equipment and improved ease of operation. Requirements for an input unit range from cost-effectiveness, precision, and compact appearance to the possibility that the input unit can be incorporated into a color display.
Various input methods have been proposed to date. Above all, an electrostatic coupling (also called capacitive coupling) method, which enables integration of an input unit into an LCD and is effective in realizing a compact and lightweight design and in reducing cost, is attracting attention.
The principle of applied voltage detection based on electrostatic coupling will be described. When a point on a LCD is touched by the tip of a stylus, a series capacitance is induced between the stylus and electrodes in the LCD via a glass substrate. Assuming that a pulsed voltage that scans each electrode is applied continually to the electrodes, when a pulse is applied to an electrode located close to the stylus, an alternating component (differential voltage) of the applied pulsating voltage is detected in a detected signal provided by the stylus. Therefore, the contact position of the stylus on the LCD is calculated by detecting the position of an electrode to which a pulse causing the detected signal to have a peak value is applied. The coordinates of the contact position shall be referred to as the pointed coordinates.
A video signal fed to an LCD is of the same type as the one fed to a CRT or the like. The period required for feeding video signals constituting one screen is referred to as a frame. The period shall also be called a vertical scanning period. Within a frame, a signal representing one horizontal line is fed continually to the vertical arrays of pixels. This period of this signal shall be referred to as a display period. The remnant of one frame from which the display period is subtracted is a period referred to as a vertical-retrace period. The vertical-retrace period originally means a period during which an electron beam returns from the right lower corner of a CRT that is a scanning end position of a screen to the left upper corner of the CRT that is a scanning start position. The same kind of video signals are fed to an LCD. The term "vertical-retrace period" is therefore often used as it is, and will also be used herein. Normally, a ratio of the vertical-retrace period to one frame is as small as several percentages.
In a conventional simple matrix type LCD, voltages that cause currents in scan electrodes are consecutively applied, vertically and horizontally, during a vertical-retrace period in order to acquire a detected signal. Thereafter, pointed coordinates of a stylus are calculated on the basis of the variations of the detection signal. At this time, the orientation of a liquid crystal in the simple matrix type LCD is dependent on the effective voltage of the applied voltage. Pulses of any voltage can be applied as detection pulses during the vertical-retrace period as long as the voltage does not affect the effective voltage greatly.
In recent years, an active matrix type LCD able to display a screen composed of a large number of pixels with high image quality and a high-quality color display has been widely adopted.
In the active matrix type LCD, scan electrodes, data electrodes, and thin-film transistors (hereinafter TFTs) are formed on a device substrate. An opposed substrate on which common electrodes are formed is opposed to the device substrate. A liquid crystal layer is interposed between the device substrate and opposed substrate. The LCD is usually placed so that the opposed substrate will face an observer in consideration of current leakage through off-state TFTs caused by strong light such as direct sunlight. When a stylus is put onto a display surface, the stylus comes into contact with the opposed substrate. The scan electrodes and data electrodes, to which pulses are applied and detected due to electrostatic coupling with the stylus, are formed on the device substrate opposed to the opposed substrate. When an LCD is placed as mentioned above, the solid common electrodes act as a kind of shield. Even if the stylus is put on the LCD, the voltage applied to the device substrate is hardly detected and the contact position of the stylus cannot be identified with sufficiently high precision.
In a conventional LCD, scan electrodes are scanned consecutively during a vertical-retrace period in order to identify a pointed y-coordinate. In this case, TFTs are turned on, and voltage in data electrodes is placed in cells. As a result, image information written during a display period is destroyed. This makes it impossible to scan scan electrodes consecutively during a vertical-retrace period in order to identify a pointed y-coordinate. Consequently, a pointed y-coordinate cannot be identified.
A signal detected by a stylus through electrostatic coupling contains various kinds of noise passing through the liquid-crystal panel. The noise deteriorates the position detection accuracy.
For identifying coordinates pointed to by a stylus, the position of the scan electrode or data electrode located closest to the position at which the stylus comes into contact with the liquid-crystal panel is detected. Resolution is therefore dependent on the spacing between adjoining electrodes. There is also a demand to indicate the coordinates with higher resolution.